*3.2. Active Power Control Strategy Applied in DERs*

When highlighting the active power control strategy implemented in small generators, it is worth pointing out existing standards, such as VDE-AR-N-4105 [52]. When referring to this standard, the active power frequency response characteristic *P(f)* and standard characteristic of the *Q*/*P*max (known also as *cos*ϕ*(P)* or *tg*φ*(P)*) for the generation unit directly connected to a low voltage (LV) network can be revealed. These characteristics are presented in Figures 1 and 2, respectively. The power frequency response characteristic assumes that for power system frequency between 50.2 Hz and 51.5 Hz. When the exceedance of frequency is detected, it is recommended to reduce active power generation from PM with a gradient of 40% PM per Hertz. At system frequencies higher than 51.5 Hz and lower than 47.5 Hz, the power generation unit will be disconnected immediately.

**Figure 1.** Standard active power frequency response characteristic *P(f)* applied for DER units integrated with an low voltage (LV) electrical network (based on Reference [52]).

When discussing relations between active power and reactive power, the characteristic of the *Q*/*P*max (known also as *cos*ϕ*(P)* or *tg*φ*(P)*) for the generation unit can be introduced. This characteristic is presented in Figure 2. It can be concluded that up to 20% of the maximum power capacity *P*max, both generation and consumption of reactive power is recommended. In the range of (0.2–0.5) *P*max, the active power generation is accepted. When exceeding half of *P*max, both generations of the active power and reactive power consumption are recommended. It may be used as a model with the capacitive power factor *cos*ϕcap. The reactive power level consumption relays on a range of *P*max. The application of reactive power consumption aims to reduce the increase in voltage that is because of the noticeable active power generation. levels.

**Figure 2.** Standard *Q*/*P*max profile (*cos*ϕ(*P*) characteristic) applied for distributed energy resource (DER) units integrated with a LV electrical network: (**a**) with regard to units with a maximum power capacity of 3.68 kVA to 13.8 kVA; (**b**) with regard to units with the maximum power capacity over 13,800 VA (based on References [52,57]).

#### *3.3. Power Quality Voltage Profile*

A common area that can be related to the mentioned control and regulation strategies and protection schemes, is the power quality. The most crucial power quality parameters are frequency variations, voltage variation, voltage fluctuation, voltage unbalance (asymmetry), voltage harmonics, interhamonics, subharmonics, direct current injection (DC) and rapid voltage changes [58,59]. A general standard related to public electrical networks is EN 50160 [60]. In conjunction with quoted standards

related to the DER, it is possible to define permissible limits of voltage changes in the connection points of the DER, as well as in other nodes of the network.

One of the critical requirements states that rapid voltage changes caused by switching operation with maximum power capacity cannot be more than 3% of the nominal voltage.

$$d\_c = \frac{|\Delta l L\_i|}{l l\_a} 100 \le 3\%,\tag{1}$$

where:


Additionally, a slow voltage changes (voltage level) in every node of distribution network consisting of DERs cannot exceed 10% of nominal voltage considering every DERs working simultaneously with maximum power capacity for 10-min aggregating time recommended by standard EN50160 [60]. Recently, in the last update of the mentioned standard [60], measurements with an aggregation time of 1-min instead of 10-min were suggested. Some investigations of the influence to aggregation interval on the assessment of photovoltaic power plant belonging to discussed VPP topology has been discussed in Reference [61].
